This disclosure relates to the field of offshore drilling and production platforms, particularly spar-type platforms. More specifically, it relates to platforms, such as spar-type platforms, that are used in conjunction with production and/or export/import risers, either in a vertical or catenary configuration. Still more specifically, it relates to a mechanism for coupling a riser configured in a catenary configuration to a platform through a set of riser guides in the platform designed for a vertical riser configuration in a way that controls the curvature of the catenary curve-configured riser at the bottom or keel of the platform so as to reduce the stresses experienced by the riser as it enters the platform.
Spar-type offshore drilling and production platforms typically comprise an elongate buoyant hull supporting a deck on which drilling and production equipment is mounted, along with other structures (e.g., crew quarters, cranes, storage structures). The hull typically comprises a plurality of buoyancy tanks surrounding a central centerwell. The buoyancy tanks may extend to down to a ballasted keel, or they may be connected to the keel by a truss structure.
The platform is typically used in conjunction with one or more risers that extend under tension from the platform to the seafloor, either vertically or in a catenary configuration. In a typical catenary-configured riser arrangement, the lower end of each riser is connected to one end of a submarine pipeline or flow line that extends along the seafloor to a second end attached to a wellhead or some other structure on the seafloor. Each riser essentially extends from the flow line upwardly into an opening in the keel of the platform then upwardly through the centerwell to an upper end that is connected to production apparatus on the deck. A vertically-configured riser, on the other hand, extends to a fixed seabed structure below or nearly below, the platform.
The risers may be supported vertically at the surface by buoyant tensioning means attached to the risers in the centerwell, and they are restrained laterally by guide means located in the centerwell. A typical buoyant-tensioned riser arrangement in a spar-type platform is disclosed in U.S. Pat. No. 6,176,646, the disclosure of which is incorporated herein in its entirety. A riser may also be supported by a mechanical tensioning mechanism, typically located on the deck of the platform, comprising a combination of hydraulic pistons and cylinders and pneumatic accumulators functioning as a nearly constant-tension spring. A third type of support for a vertical riser is a fixed attachment to the hull of the platform, requiring the elasticity in the riser system to be supplied along the riser or at the seafloor. Regardless of the particular means employed for providing vertical support, the platform must be equipped with an array of vertical riser guide structures that constrain the lateral motion of the vertical riser as it passes through the platform hull.
One type of riser configured as a catenary is made of steel and is commonly called a “steel catenary riser” (SCR). The SCR is typically a length of steel pipe that defines a curved catenary configuration as it rises from the seabed to the keel of the platform. One important design consideration in the use of SCRs is the need to minimize static and dynamic stresses from bending moments on the riser at or near the keel. Specifically, an SCR must undergo a transition from a catenary curve to a straight linear (i.e., vertical) configuration. This transition produces a static bending moment in the SCR, which, in turn, creates stresses. In addition, the motion of the floating platform creates cyclical stresses on the riser that, over time, can result in fatigue and failure. Accordingly, it is desirable to reduce the static bending moments and dynamic stresses to the greatest extent possible. This can be done, for example, by running the SCR through a bend limiter at the keel, such as is disclosed in the aforementioned U.S. Pat. No. 6,176,646. The bend limiter, as its name implies, limits the degree of riser bending at the keel, whereby the riser configuration makes a gradual, as opposed to an abrupt transition from a catenary curve to a straight vertical line. Conventional bend limiters typically comprise a length of steel pipe having an inside diameter large enough for passage of the riser. While such prior art bend limiters do result in some limitation of the bending moment in the riser, they are fixed in geometry and cannot optimally support the dynamic motion of the riser relative to the platform. Furthermore, conventional bend limiters with fixed geometries typically cannot be run through the vertical riser guides in the centerwell of the platform, and thus the prior art bend limiters must be secured to or within the hull, independently of the available vertical riser guides.